1. Field of the Invention
The present invention relates to a multiple image forming apparatus for forming different images respectively on different recording members and transferring said images onto a same recording medium to obtain a multiple overlaid image.
2. Related Background Art
At first reference is made to FIG. 29 for explaining the function of a conventional color copying machine utilizing a single photosensitive drum.
In a main body 1200 of the one-drum color copying machine, there are provided a photosensitive drum 1201 rotated in a direction a, a charger 1202, a laser unit 1203, an exposure charger 1204, a surface potential sensor 1205, and color developing units 1206 for yellow, 1207 for magenta and 1208 for cyan.
The copying machines explained above functions in the following manner. The charger 1202 charges, by corona discharge, the surface of the rotating photosensitive drum 1201. A laser beam, emitted by the laser unit 1203, is projected onto the drum 1201 through mirrors. Said laser unit is provided with an unrepresented polygon mirror which is rotated to deflect the laser beam in the main scanning direction to form a main scanning line. Said laser beam is turned on and off to form an electrostatic latent image, in the form of pixels, on the surface of the photosensitive drum 1201. Subsequently the latent image is rendered visible by the deposition of color toner in one of the color developing units 1206, 1207, 1208. The obtained toner image is transferred, by means of a transfer charger 1209, onto a recording sheet 1210 supplied by a feed roller 1211 from a sheet cassette. A difference from a black-and-white copying machine lies in a fact that the sheet 1210 is supported on a support drum 1215 and rotated in a direction b. Said support drum 1215 is provided for transferring the toner images in the order of yellow, magenta and cyan onto the sheet 1210, and the peripheral speed of said drum is same as that of the photosensitive drum 1201.
After the transfer of three primary colors, the recording sheet is peeled off from the support drum 1215 and is transported to a heat fixing station 1214, and the sheet subjected to image fixation therein is discharged by a discharge roller 1213 to a copy tray 1212.
However, in terms of the copying speed, a system employing plural photosensitive drums as shown in FIG. 3 is more advantageous than the conventional structure shown in FIG. 29. Nevertheless the system with four photosensitive drums shown in FIG. 3 is still associated with problems to be solved, such as the registration of different color images, necessity for frame memories depending on the distance of the photosensitive drums, and correction for fluctuation in the sensitivity of plural photosensitive drums.
The aberration in the registration of different color images is evaluated by the observation with naked eyes, and the tolerance therefor is generally considered in the order of 100 .mu.m.
However the control of said image registration becomes difficult if a sheet feed signal for a succeeding recording sheet is released before the start of image formation on the last photosensitive drum. For this reason the sheet feed signal for the next recording sheet is released after the start of image formation on the last photosensitive member, and high-speed printing cannot be achieved because of this fact.